Am I alone in really not seeing what Power Sources in general bring to the party? So far, I haven't ever found myself thinking "I'm looking for a class that's really Primal", or "Well, I would be interested in playing that class if it wasn't so darn Arcane".
In the end, Power Sources are only a very loose linking theme, that provide almost nothing mechanically across the classes linked to them. The true differentiation occurs at the Class level, and frankly, if each individual class defined itself and the origins of its powers purely in its own terms, you'd lose little if anything compared to the current system, whilst freeing up classes from having to be shoehorned into some clumsy over-arching concept.
I know what you mean, but consider the following:
Party 1 - Fighter, Rogue, Warlord, Wizard
Party 2 - Warden, Barbarian, Shaman, Druid.
Whilst both parties cover the four roles well, the implications and links between the members of the group are going to make them totally different in play. I think the power sources are worth as much as you're prepared to put in to them.
For example, I'm playing a Druid in one game. The local government is a Theocracy, and whilst other members of the group (a fighter, a ranger, a rogue and a bard) have their problems with them and their plans, my Druid is alone is opposing them on a more fundamental level - they are consorting with Far Realms powers and as such I've declared to the members of the party that I want to burn their Cathedral to the ground, if possible.
It's leading to some inter-party conflict, but in a good way. We've all more-or-less pulling in the same direction, but we've got some radically different ideas about how to go about it. My Druid is all for killing Theocracy priests out of hand, surrendered or not, but he's been talked round a couple of times by other PC's. Sooner or later, though, we'll another chance to make a moral decision. It's a fun game, and the power sources of the various characters are an absolutely key part of that.